Fax machines have long been used to transmit images from one location to another via a voice-grade telephone line. Such machines typically comprise a controller, a document scanner for converting the image of a document into a serial data message (and vice versa) and a modulator/demodulator, i.e. "modem", for converting the serial data into a series of tones (and vice versa) for communication on the telephone line. The telephone line may be a wire or optical cable, or may include a wireless connection such as a microwave, a satellite link or a cellular system. Generally, a sending fax machine will scan a sheet of paper and convert its image into a code for transmission over the telephone line. The receiving machine reconverts the codes and prints a facsimile of the original. Some fax modems can send and receive faxes, others can only send but not receive faxes.
In general, the various phases of a facsimile transmission are as follows:
Phase A is the call establishment phase in which the telephonic communication is established between a calling station, i.e. the station initiating the facsimile telephone call, and a called station, i.e. the station receiving the facsimile telephone call. Telephonic communication is established in the same way as in any telephone call: upon receiving a dial tone, the calling station provides the phone number of the called station to the telephone switching system, and, in response, the switching system establishes a circuit connecting the calling station to the called station and provides a ringing signal to the called station. The called station answers the ringing signal by going off-hook, i.e. the called station effectively lifts the receiver. For a facsimile transmission, as is well known, the calling station and the called station must have the capability of transmitting and receiving facsimile messages. Typically, this capability is provided by a transmitting fax machine and a receiving fax machine.
Phase B is a pre-message procedure phase or a negotiation phase between the calling station and the called station for identifying and selecting facsimile data encoding capabilities. Whichever station is to receive the facsimile message transmits a digital identification signal (DIS) to the transmitting fax machine. The DIS contains information indicating the data encoding capabilities of the receiving fax machine. The transmitting fax machine uses the information in the DIS to determine the appropriate data encoding format.
Phase C is the actual message transfer phase and comprises two simultaneous phases. Phase C1 is the "in-message" procedure phase, which controls the transfer of data between the transmitting fax machine and the receiving fax machine. This includes synchronization, error detection and correction, and line supervision. Phase C2 is the actual data transmission phase in which encoded data is transferred by the transmitting fax machine to the receiving fax machine under control of the signals generated in the C1 phase.
Phase D is the post-message procedure phase in which information is transferred regarding the end-of-message signaling, confirmation signaling, and end of facsimile signaling.
Phase E is the call-release phase in which the telephonic communication between the calling station and the called station is terminated.
The cost of sending a facsimile message long distance includes the transmission cost of the telephone call, which cost is directly related both to the size of the message being sent and to the transmission speed at which the message is sent. In some cases the cost of sending a facsimile message is also related to the time of day during which the facsimile telephone call is placed. To reduce costs, the user may send the facsimile message during low-cost periods, such as at night, when the demand for telephone lines is minimal. Thus, the telephone rate is decreased and the cost of sending the facsimile message is correspondingly decreased. The cost can also be reduced by minimizing the duration of the facsimile telephone call.
Although fax machines have gradually become faster and more efficient through the years, current standards and technology still limit the fax transmission speed. Fax machines in the 1960's (Group I) were analog units capable of transmitting a page within four to six minutes. These were replaced by fax machines introduced in the mid-1970's (Group II) which were capable of transmitting a page within two to three minutes. Group III fax machines are capable of transmitting at a rate of 9 to 20 seconds per page. Virtually all fax machines in the market today are Group III machines. Newer fax machines (Group IV) offer an even faster transmission speed of approximately 5 seconds per page. However, Group IV fax machines require access to digital telephone lines in order to achieve these high transmission speeds.
The cost of transmission can be reduced by minimizing the duration of the facsimile telephone call. One such method for minimizing the duration of the telephone call is to compress the data prior to transmission. Early facsimile machines used a basic data encoding technique, known as Modified Huffman (MH) encoding. This form of encoding permits compression of the facsimile message, with a corresponding decrease in the duration of the facsimile telephone call. Other data encoding techniques were developed for newer facsimile machines, although newer facsimile machines continue to retain MH encoding capabilities to enable facsimile communication of the older design. A Modified Read (MR) encoding permits two dimensional data encoding, which permits a higher degree of data compression than MH encoding permits. Both MH and MR encoding are described in standards established by the International Telegraph and Telephone Consultative Committee (CCITT) and published as Terminal Equipment and Protocols for Telematic Services (1989). The MH and MR data encoding are described in recommendation T.4 of the CCITT standard.
A third type of data compression is the Modified Modified Read (MMR) encoding, described in recommendation T.6 of the CCITT standard, which permits two dimensional encoding of low- or high-resolution facsimile messages.
Other types of data compression, such as Joint Bi-level Image Experts Group (JBIG), offer even greater compression capabilities than MMR but have not generally been integrated into fax machines. The JBIG data encoding is described in recommendations T.82 and T.85 of the CCITT standard. Generally, JBIG compresses binary (one bit/pixel) images. JBIG provides a compatible progressive/sequential coding, meaning that a progressively coded image can be decoded sequentially, and the other way around. All bits in the image, which is compressed using the JBIG format, before and after compression and decompression will be exactly the same. For further information concerning the use of data compression techniques in facsimile, see Gilbert Held, The Complete Modem Reference, (2d Ed.), pp. 237-256 (Wiley 1994) and Andrew Margolis, The Fax Modem Sourcebook, pp. 111-134 (Wiley 1995), both of which are incorporated herein by reference.
Another method for reducing the length of the telephone call is to increase the rate or transmission speed at which the message is sent. Most fax machines typically transmit data at a rate of 9.6 and/or 14.4 kilobits per second (kbps). Some modems can transmit fax at rates of 28.8 kbps or higher, but these are generally not used in the current installed base of fax machines.
Alternatives to the use of fax machines are well known. Many personal computers now have a modem and the capability of generating the serial data message of a facsimile internally without the need to scan a document. These computers also have the capability of executing the communication protocol required to transmit the message to a remote fax machine. These computers, however, transmit data using communications techniques employed by fax machines which are not as efficient as various other data communication methods. In addition, these computers transmit only computer files that are stored in digital format and are not useful for processing paper documents.
Many modern personal computers also have the capability of transmitting e-mail to a remote computer using various packet-switched digital communication networks such as the Internet. E-mail, however, is of no use if the message to be sent is initially in the form of document that has to be scanned.
From the foregoing it will be apparent that fax machines typically encode data somewhat inefficiently, transmit data at relatively slow rates (compared to computer generated facsimiles or e-mail), and use circuit-switched rather than packet-switched networks. While e-mail may be a more efficient communication medium, it does not provide any means for transmitting pre-existing documents other than re-creating them anew as e-mail documents. Moreover, there are currently millions upon millions of fax machines already in operation.
Thus, it is desirable to achieve efficiencies of packet-switched digital communication networks while continuing to use existing fax machines as well as to introduce fax machines which communicate over such networks. There is also a need for cost savings in fax transmission and, in particular, there is a need to reduce long distance telephone charges associated with faxes.